1. Field of the Invention
The present invention is concerned with catalytic structures comprising a fired coating of a catalytic ink adhered to a support member, such as a ceramic support member. The catalytic structure may be, for example, a sensor in which the catalytic coating catalyzes a gas phase reaction such as oxidation, the extent of which is monitored by temperature sensing means forming part of the sensor.
2. Related Art
Standard thick film technology involves screen printing a paste or ink that contains three components: an organic vehicle, a functional material (e.g., precious metal), and a glass frit or oxide. The organic vehicle provides a more or less homogeneous suspension of the functional material and imparts to the paste or ink a viscosity suitable for screen printing. The functional material is the ingredient that supplies the property needed for a particular application, e.g., platinum for an electrical or electronic circuit, or catalyst particles for promoting a chemical reaction such as oxidation. The glass frit supplies the bond between particles of the functional material (cohesion) and between the functional material and the substrate on which it is coated (adhesion). However, glass frits typically contain components such as boron, bismuth or lead which have negative effects on catalyst activity, selectivity and stability. Therefore, these materials are not of utility for many catalyst compositions.
U.S. Pat. No. 4,185,131, issued Jan. 22, 1980 to G. J. Goller et al and entitled "Screen Printing Method For Making an Electrochemical Cell Electrode", discloses fuel cell electrodes made on a continuous basis using screen printing. A wet floc of a co-suspension of carbon and hydrophobic polymer is dried and reduced to a fine powder and resuspended in an inking vehicle. The ink is then screen printed onto a porous substrate and the ink vehicle removed by heating. The resulting coating is compacted, sintered and then catalyzed, such as with platinum. Catalyzation of the sintered coating is stated to be carried out by the methods described in Katz et al U.S. Pat. Nos. 3,932,197 or 3,979,227.
U.S. Pat. No. 4,229,490, issued Oct. 21, 1980 to S. N. Frank et al and entitled "Novel Method For Catalyst Application to a Substrate For Fuel Cell Electrodes", discloses applying a platinum black plus graphite catalyst to a substrate by screen printing it onto a thin, carbonized paper-like substrate.
U.S. Pat. No. 5,273,779, issued Dec. 28, 1993 to I-Cherug Chen et al and entitled "Method of Fabricating a Gas Sensor and the Product Fabricated Thereby", discloses applying a buffer layer onto a substrate and applying at least one gas sensing layer atop the buffer layer. A pair of electrodes is disposed on the gas sensing layer and a catalytic layer is coated onto the gas sensing layer by a spin-casting process, using centrifugal force, to form thin and evenly deposited layers on the substrate.
The book Principles of Chemical Sensors by Jiri Janata, Plenum Press, New York, N.Y., 1989 (ISBN 0-306-43183-1), discloses at pages 51-53 thereof, catalytic gas sensors stated to be useful for the detection of sub-threshold concentrations of flammable gases in ambient air and finding use as a safety expedient in mining operations. A simple sensor is illustrated in FIG. 2-10 showing a support bead such as a pellet of thoria/alumina coated with a porous catalytic metal, palladium or platinum, and having a platinum coil embedded within the pellet. The coil is stated to act both as a heater and as a resistance thermometer but any type of temperature sensor is stated to be usable. When a combustible gas reacts at the catalytic surface, heat evolved from the reaction is stated to increase the temperature of the pellet and of the platinum coil, thus increasing its resistance. The latter is measured, for example, by use of a Wheatstone bridge.
An article entitled Thick Film Pellistor Array With a Neural Network Post-Treatment by H. Debeda et al was published by Elsevier Science S.A. in Sensors and Actuators B 26-27 (1995) at pages 297-300. This article discloses pellistor gas sensors and also notes that a typical pellistor consists of a platinum wire supported in an alumina bead impregnated with a finely dispersed noble metal such as palladium. As illustrated in FIG. 2 and discussed in paragraph 2, screen printed pellistors are made by screen printing a platinum resistor on one side of an alumina substrate and depositing a catalytic layer on the other side of the substrate, which catalytic layer is then fired. The method of depositing the catalytic layer is not specified.